Giải thích các bước giải:
\(\begin{array}{l}
\left\{ \begin{array}{l}
\frac{a}{5} = - \frac{b}{3}\\
\frac{a}{5} = \frac{c}{2}\\
2a + b + 3c = 39
\end{array} \right. \to \left\{ \begin{array}{l}
b = \frac{{ - 3a}}{5}\\
c = \frac{{2a}}{5}\\
2a - \frac{{3a}}{5} + \frac{{6a}}{5} = 39
\end{array} \right.\\
\to \left\{ \begin{array}{l}
a = 15\\
b = - 9\\
c = 6
\end{array} \right.
\end{array}\)